Polypropylene fiber is traditionally stabilized with a blend of selected phenolic antioxidant, selected phosphite and selected hindered amine light stabilizer. This formulation generally provides adequate processing, heat and light stabilization performance, but does not provide adequate gas fade resistance which is needed to maintain color properties during storage and end-use application. There is a long-felt need in the marketplace for a stabilizer system which can prevent this gas fading and color formation associated with the use of phenolic antioxidants. Gas fading is known in the industry as a discoloration resulting from the exposure of plastic articles to an atmosphere containing oxides of nitrogen.
The components of the instant stabilizer system for polypropylene fibers are generically well-known as stabilizers for a host of organic and polymeric substrates. The components of the instant stabilizer system for polypropylene fiber are a specific combination of selected 2,2,6,6-tetramethylpiperidine hindered amines and N,N-dialkylhydroxylamines, in the absence or essential absence of a phenolic antioxidant. This instant stabilizer formulation provides unexpectedly superior gas fade resistance, and heat and light stability performance properties to the polypropylene fibers which are notoriously difficult to stabilize effectively. The instant phenolic free antioxidant stabilizer system provides the best overall stabilization for polypropylene fiber. Discoloration of polypropylene fibers, when exposed to an atmosphere containing oxides of nitrogen, i.e. gas fading conditions, encountered with stabilizer systems containing phenolic antioxidants, makes such systems unacceptable in this important property even though in other performance criteria the phenolic antioxidants perform adequately.
The hindered amines are a very important class of light and thermal stabilizers based on compounds having a 2,2,6,6-tetramethylpiperidine moiety somewhere in the molecule. These compounds have achieved great commercial success and are well-known in the art.
N,N-Dialkylhydroxylamines also are known in the an as seen in U.S. Pat. Nos. 4,590,231; 4,782,105; 4,876,300 and 5,013,510. These compounds are useful as process stabilizers for polyolefins when used alone or in combination with phenolic antioxidants and/or other coadditives, particularly as taught in U.S. Pat. No. 4,876,300. Although U.S. Pat. No. 4,876,300 teaches genetically that N,N-dialkylhydroxylamines can be used in combination with phenolic antioxidants, hindered amines, phosphites, UV absorbers and other additives, there is no specific disclosure that polypropylene fibers can be beneficially stabilized by the instant specific combinations of selected hindered amines and the instant specific N,N-dialkylhydroxylamine. Thus the instant invention is essentially a selection from within the broad generic scope of U.S. Pat. No. 4,876,300.
U.S. Pat. No. 4,876,300 is directed primarily to polypropylene compositions in general with only one example being directed to polypropylene fiber. In said example, the specific hydroxylamine used is N,N-di(hydrogenated tallow)hydroxylamine where the alkyl moiety is a mixture of tetradecyl, hexadecyl, heptadecyl and octadecyl groups as set forth in U.S. Pat. No. 4,876,300.
However, the instant composition is distinguished from the compositions of the prior art in several important aspects listed below:
1. Hindered phenolic antioxidants plus phosphites combinations have generally poor gas fade resistance;
2. Phosphites alone lack adequate process, thermal and light stabilization efficacy; and
3. Phosphites plus hindered amines lack adequate process stabilization.
4. Substitution of the N,N-di(hydrogenated tallow)hydroxylamine described in U.S. Pat. No. 4,876,300 by the N,N-dialkylhydroxylamine made by the direct oxidation of N,N-di(hydrogenated tallow)amine by the process of U.S. Pat. Nos. 5,013,510 or 4,898,901 leads to unexpectedly superior results in the binary compositions with selected hindered amine light stabilizers compared to the results reported in U.S. Pat. No. 4,876,300.
The instant combination of stabilizers provide all of the required requisites of gas fade resistance and process and thermal stability.